1. Field of the Invention
The present invention relates to an electron-emitting device, and a method of preparing it.
2. Related Background Art
Hitherto known as a device achievable of emission of electrons with use of a simple structure is the cold cathode device published by M.I. Elinson et al (Radio Eng. Electron. Phys., Vol. 10, pp. 1290-1296, 1965). 
This utilizes the phenomenon in which electron emission is caused by flowing an electric current to a thin film formed with a small area on a substrate and in parallel to the surface of the film, and is generally called a surface conduction electron-emitting device.
This surface conduction electron-emitting device that has been reported includes those employing a SnO2(Sb) thin film developed by Elinson et al. named in the above, those employing an Au thin film (G. Dittmer, “Thin Solid Films”, Vol. 9, p. 317, 1972), those employing an ITO thin film, (M. Hartwell and C.G. Fonstad, “IEEE Trans. ED Conf.”, p. 519, 1975), and those employing a carbon thin film [Hisashi Araki, et al. “SHINKU” (Vacuum), Vol. 26, No. 1, p. 22, 1983]  (Hisashi Araki Et al. “SHINKU” (Vacuum), Vol. 26, No. 1, p. 22, 1983). 
Typical device constitution of these surface conduction electron-emitting devices is shown in FIG. 38. In FIG. 38, the numerals 19 and 20 denote electrodes for attaining electrical connection; 21, a thin film formed using an electron-emitting material; 23, a substrate; and 22, an electron-emitting region.
In these surface conduction electron-emitting devices, it has been hitherto practiced to previously form the electron-emitting region by an energizing heat treatment, called “forming”, before effecting the electron emission. More specifically, a voltage is applied between the above electrode 19 and electrode 20 to energize the thin film 21 to bring the thin film 21 to be locally destroyed, deformed or denatured owing to the Joule heat thereby generated, thus forming the electron-emitting region 22 kept in a state of electrically high resistance to obtain an electron-emitting function.
What is meant by the above state of electrically high resistance is a discontinuous state of a film partly having cracks of 0.5 μm to 5 μm on the thin film 21 and having the so-called island structure inside the cracks. What is meant by the island structure is the structure of a film in which fine particles generally having a diameter of several ten angstroms to several micrometers are present on the substrate, and the respective fine particles are spatially discontinuous and electrically continuous.
Hithertofore, in the surface conduction electron-emitting devices, a voltage is applied to the above high-resistance discontinuous film by the electrodes 19 and 20 to flow an electric current to the surface of the device, so that the electrons are emitted from the above fine particles.
However, the forming according to the conventional energizing heat treatment as mentioned above have involved the problems as follows:
(1) In carrying out the energizing heating, it sometimes occurs that the thin film is peeled because of the difference in coefficient of thermal expansion between the substrate and the thin film. This provides limitations in upper limit of heating temperature, materials for the substrate, and combination by selection of materials for the thin film.
(2) In carrying out the energizing heating, the substrate also is locally heated, therefore sometimes resulting in occurrence of fatal cracking therein.
(3) Degree of the changes of a film owing to the energizing heating, as exemplified by the degree of local destruction, deformation or denaturing, tends to become irregular among a plurality of devices formed in the same substrate, and also the site at which changes may occur tends to be not fixed.
For this reason, when functioned as an electron-emitting device, irregularity in the shape of beams of emitted electrons has been seen for each device.
(4) A relatively large electric power is required until the forming is completed. For this reason, an electric source of large capacity is required when a number of devices are formed on the same substrate and the forming is carried out simultaneously.
(5) A relatively long period of time is required for conventional forming processes that start with the energizing heating and end with cooling. For this reason, an excessively long time is required for carrying out the forming of a number of devices.
Because of the problems as set out above, the surface conduction electron-emitting devices have not been positively applied in industrial fields, notwithstanding their advantages that the device has simple construction.